HELMINTH-VERTEBRATE RESERVOIR

In this reservoir type, the bacteria are in a complex relationship involving both vertebrate hosts and at least one helminth species capable of infecting that vertebrate. This reservoir type is characteristic of only a few bacteria.

The best studied bacteria in this reservoir type are members of the genus Neorickettsia, obligate intracellular parasites, with at least four species involved in a relationship with both trematodes and mammals (Rikihisa et al. 2004). The best-known example among wildlife is N. helminthoeca, the causative agent for salmon poisoning disease; this rickettsia uses both mam­malian carnivores and the digenetic trematode Nanophyetus salmincola (Foreyt 2001). Another example among wildlife is N. (Ehrlichia) risticii, which can use at least two species of trematodes (Acanthatrium oregonense, Lecithodendrium sp.) and infects the trematode definitive hosts, including bats, coyotes, and some birds (swal­lows); the rickettsia also causes Potomac horse fever among domestic horses (Pusterla et al. 2000, 2003; Gibson et al. 2005). Neorickettsia sennetsu, the cause of human sennetsu rickettsi- oses, as well as the SF agent, also a Neorickettsia sp., both occur in the trematode Stellantchasmus falcatus (SF) and infect humans (Fukuda and Yamamoto 1981, Rikihisa et al. 2004).

In an interesting possibility, Brucella sp. (B. pinnipedialis?) has been recovered from Parafilaroides spp. lungworms from a harbor seal (Phoca vitulina richardsi) (Garner et al.

1997). Brucella spp. also were isolated from the lungworms (Pseudalius inflexus) of a harbor porpoise (Phocoena phocoena) (Dawson et al. 2008). Authors of both studies propose that brucellae might be transmitted among pinni­peds by infected lungworms. However, there has been no further work to evaluate the likeli­hood of this reservoir type being used among these brucellae.

Control strategies logically could be the same as those directed toward control of helminth infections, with an attempt being made to disrupt their life cycle through habitat management, bio­logical control, or toxic chemicals. In the event of an endangered or high-profile species, one could immunize or treat individual hosts.

Neorickettsia Helminthoeca

causative agent Neorickettsia helmin- thoeca are coccoid, obligate intracellular, gram­negative bacteria (Family Rickettsiaceae; App.1: Table 7) (Foreyt 2001), antigenically and geneti­cally related to Ehrlichia spp. (Rikihisa 1991). Another closely related organism, the Eloko- min fluke fever (EFF) rickettsia, may be a strain of N. helminthoeca (Frank et al. 1974a, 1974b).

host range and geographic distribution Neorickettsia helminthoeca causes mortality in coyotes (Canis latrans), red foxes (Vulpes vulpes), and domestic dogs; wolves (Canis lupus) and other canids also may be susceptible (Foreyt 2001). The geo­graphic range of the parasite is tied to the distribution of infected snails of the trema­tode Nanophyetus salmincola (Rikihisa et al. 2004) and includes the coastal portions of Washington, Oregon, and northern California (Foreyt 2001). This bacterium also has been reported recently from domestic dogs in Brazil (Headley et al. 2006).

reservoirs and transmission Neorick­ettsia helminthoeca is found in all stages of the trematode Nanophyetus salmincola (Nyberg et al. 1967, Millemann and Knapp 1970, Knapp and Millemann 1981). The snail Oxytrema silicula is the only known first intermediate host of the trematode parasite, but many fish, including salmon and trout, as well as the Pacific giant salamander (Dicamptodon ensatus), may serve as second intermediate hosts (Gebhardt et al. 1966, Foreyt 2001). Fish are infected in fresh­water by trematode cercariae and retain the trematode and rickettsial infection throughout their ocean migration before returning to fresh­water up to 3 years later (Millemann et al.

clinical effects Although a variety of definitive hosts can be infected by N. helminthoeca-infected fish, only foxes, coyotes, and domestic dogs die from N. helminthoeca (Foreyt 2001). Among infected coyotes and dogs, the typical course is a high fever after a 5- to 7-day incubation period, fol­lowed by hypothermia and death about 7 to 10 days after clinical symptoms begin; other effects are anorexia, marked weight loss, and depression (Foreyt et al. 1987, Foreyt 2001). About 90% of infected dogs, foxes, and coyotes die if untreated (Knapp and Millemann 1981) The Elo- komin fluke fever (EFF) strain of N. helminthoeca can infect black bears (Ursus americanus) and rac­coons (Procyon lotor) as well as canids; bears and raccoons are refractory to other N. helminthoeca strains (Knapp and Millemann 1981). The EFF strain also has high morbidity but low mortal­ity for dogs, has a longer incubation period, and elicits a reduced febrile response in dogs (Farrell et al. 1973, Knapp and Millemann 1981).

Suspicion of N. helminthoeca in domestic dogs often results from finding operculated trematode eggs in feces following ingestion of infected fish; however, the trematode eggs themselves are not conclusive (Foreyt 2001).

Diagnosis of N. helminthoeca often is based on finding intracytoplasmic rickettsial bodies in mononuclear phagocytes of suspect animals after staining fluids from enlarged lymph nodes with Giemsa (Foreyt 2001).

population effects Although mortal­ity among untreated canids is 90% or more, effects of this parasite on wild fox and coyote populations of the enzootic habitats are not known (Foreyt 2001). There currently is no evi­dence of any serious population impacts being caused by this parasite among wild canids.

special problems Despite the high mor­tality in canids, no special problems have been identified for this parasite. This may partly reflect the very small geographic range of the parasite and the lack of information for naturally infected populations.

control Among captive animals, several antibiotics, including sulfonamides, can be used to treat infections (Gorham and Foreyt

1998). Adult N. salmincola flukes are controlled in coyotes and dogs by the drug praziquantel (Foreyt and Gorham 1988a); however, praziqu­antel is not effective against Nanophyetus meta- cercariae in Chinook salmon (Oncorhynchus tsawytscha) (Foreyt and Gorham 1988b). No vaccines are available.

Metacercar iae ca n rema in v iable for months in rotting fish carcasses and can infect canids eating decomposed fish; how­ever, N. helminthoeca will not be transmit­ted to the canids under these circumstances (Foreyt 2001). Helminth control is not consid­ered feasible as a primary strategy with this disease (Foreyt 2001).